1. Field of the Invention
The present invention relates to a method for producing a metal laminate having a film (which film is hereinafter referred to as a thermoplastic liquid crystal polymer film) made of a thermoplastic polymer capable of forming an optically anisotropic melt phase (which thermoplastic polymer is hereinafter referred to as a thermoplastic liquid crystal polymer). The metal laminate obtained by the method of this invention has excellent properties such as low moisture absorbability, heat resistance, chemical resistance and electrical properties deriving from the thermoplastic liquid crystal polymer film and also has excellent thermal dimensional stability. Therefore, it is useful as a material for a circuit board or a base material which requires thermal dimensional stability.
2. Description of the Related Art
In recent years, the demands for scale reduction and weight reduction in portable electronic devices for mobile communication and others are becoming high, and an expectation for high density mounting is becoming increasingly strong. In accordance therewith, multi-layered wiring boards, reduced wiring pitches, fine via holes, and small-size multiple-pin IC packages are being developed, and also the scale reduction and surface mounting of passive elements such as capacitors and resistors are taking place along with this. Particularly, the technique of making these passive components directly mounted on a surface or built in an inside of a printed wiring board or the like can achieve high density mounting and an improvement in reliability. As a result, the dimension accuracy of the wiring boards or accuracy of wiring pitches, is highly required, and further there is a demand for thermal dimensional stability.
The thermoplastic liquid crystal polymer film having excellent properties such as low moisture absorbability, heat resistance, chemical resistance and electrical properties is under rapid development as electrical insulation materials which improves reliability of printed wiring boards and the like.
Conventionally, in producing a metal laminate for a circuit board such as a printed wiring board, a thermoplastic liquid crystal polymer film, cut to a predetermined size, and a metal foil are placed in superposition between two hot platens with the use of a vacuum hot press device, and thermally press-bonded in a vacuum state (batch type vacuum hot press lamination). At this time, if molecular orientation ratio of the thermoplastic liquid crystal polymer film before press-bonding has a value of approximately 1 with an aspect ratio of mechanical strength, a metal laminate having satisfactory dimension stability can be obtained. However, since the vacuum hot press lamination is a sheet-type production method, the period of time for superposing the materials, the period of time for one pressing operation, the period of time for taking out the materials after pressing, and the like will be long. So, the production speed per one sheet of the metal laminate may be slow and the production cost may be increased. Moreover, if the equipments are improved so that a large number of sheets can be produced at the same time in order to improve the production speed, then the equipments will be large in scale, disadvantageously leading to high equipment cost. Accordingly, there is a demand for solving this problem and developing a continuous production method capable of providing metal laminates at a low production cost.
EP 0507332 A2 discloses a process for producing laminates, which comprises placing in layers at least one film comprising a liquid crystal polymer capable of forming an optically anisotropic melt phase and at least one metal foil and pressing the resulting layers by passing them through a nip between press rolls at a temperature in the range from 80° C. below the melting point of said liquid crystal polymer to 5° C. below the said melting point.
EP 0507332 A2 discloses conditions for improving a bond strength between a thermoplastic liquid crystal polymer film and a metal foil, and improvement of the mechanical strength of layer of the thermoplastic liquid crystal polymer film in the resultant metal laminate are taken into consideration. However, EP 0507332 A2 is silent with the dimension stability of the resultant metal laminate. In other words, in the method of EP 0507332 A2, the temperature condition at the time of the thermal press-bonding of the thermoplastic liquid crystal polymer film and metal foil between the press rolls is taught, but residual strain of the film before thermal press-bonding is not considered. When thermoplastic liquid crystal polymer film having the residual strain is thermally press-bonded to the metal foil, a form of the film becomes poor, and thus the resultant metal laminate does not have enough dimensional stability and flatness. For this reason, it is difficult to continuously and stably obtain flat metal laminates with excellent dimensional stability by the method of EP 0507332 A2.